1. Field of the Invention
The present invention relates to a zoom lens optical system used for a photographing device such as classic or video cameras and, more particularly, to a zoom lens optical system with five lens groups designed to achieve an ultrawide angle of view and high zooming ratio.
2. Description of the Related Art
Digital still cameras (DSCs) are currently receiving considerable attention as photographic devices. With the advancement of technology, DSCs tend to offer a wider angle of view, a higher zooming ratio, and a more compact design. To achieve these features, the DSCs commonly use a three-group zoom lens with a half field angle of 32° at wide angle position and 3× zooming ratio. The zoom lens includes three lens groups having negative, positive, and negative refractive powers, respectively.
In a conventional single-lens reflex (SLR) camera using 35 mm film, due to competitive development of a standard zoom lens with enhanced capabilities, a focal length (angle of view) is shortened to 24 mm at a wide angle position at half field angle of about 42° from 28 mm at half field angle of about 37° and a zoom ratio is increased to about 8× at about a 24-200 mm focal length from about 7× at a 28-200 mm focal length.
To achieve a wider angle of view and a higher zooming ratio for a digital camera as well, a zoom lens including five lens groups with positive, negative, positive, positive, and positive refractive powers has been proposed. Examples of a five-group zoom lens are disclosed in Japanese Laid-Open Patent Application Nos. 2002-98893, 2005-17915, 2005-55625, and 2005-70437.
A zoom lens optical system proposed in the Japanese Laid-Open Patent Application No. 2002-98893 has a high zooming ratio from 7× to 10× and about 37° of a half field angle at a wide angle position. The zoom lens optical system provides a wide angle of view comparatively and a high zooming ratio; the zoom lens optical system does not, however, provide a sufficiently wide angle of view.
Another example of a zoom lens optical system disclosed in the Japanese Laid-Open Patent Application No. 2005-17915 has a zooming ratio of about 5× and about 39° of a half field angle at a wide angle position. The zoom lens optical system provides a wide angle of view comparatively; however, the zoom lens optical system does not provide a sufficiently wide angle of view and high zooming ratio.
Another example of a zoom lens optical system proposed in the Japanese Laid-Open Patent Application No. 2005-55625 has a wide angle of view because a half field angle at a wide angle position is about 39° but a low zooming ratio of 3.9×.
Yet another example of a zoom lens presented in the Japanese Laid-Open Patent Application No. 2005-70437 has a half field angle of about 40° at a wide angle position and a zooming ratio of 7×. The zoom lens optical system has a wider angle of view and higher zooming ratio than the zoom lens optical systems proposed in the above-cited references but does not provide satisfactory wide angle of view and zooming ratio in view of the current tendency toward a wider angle of view and higher zooming ratio.